1. Field of the Invention
The invention relates to a process for the injection moulding of thermoplastic polymers.
2. Discussion of Prior Art
In such a process, molten polymer material is injected into one or more cavities defined by separable mould parts, allowed to cool and solidify and extracted after the mould parts are separated. Thermoplastic polymers shrink on cooling and this causes sink marks apparent on the surface at regions where the article is thicker than in neighbouring regions.
Sink marks seriously detract from the appearance of moulded thermoplastic articles and efforts have been made to obviate them. One process injects high pressure air into the partly solidified melt and it is found that the air forms channels within the melt and permeates principally to relatively thick regions of the article. Here the air is locked under high pressure and exerts an outward force which inhibits external sinking as the melt cools.
A difficulty with the above described method is that air flow through the melt is restricted and often cut off at narrow regions of the melt since these usually solidify first. The method is therefore not suitable for some forms of article and in particular may not be suitable for multi-cavity moulds for moulding multiple parts since variation in air passage profile would cause inequality in part material.
An alternative method is shown in patent specification U.S. Pat. No. 3,345,687 where air under pressure is introduced between one mould surface and the semi-molten polymer of the part being moulded. This method uses low air pressure. For current commercial products and polymers very much greater air pressures would be required and the use of higher pressures introduces the difficulties that air can leak from between the part being moulded and the mould parting line and polymer can be forced back through the cavity gate point. The present invention seeks to provide an improved injection moulding process.